1. Field of The Invention
The invention relates to a feed nozzle and process for atomizing liquid feed to a refinery process such as a catalytic cracking reactor.
2. Description of Related Art
Refiners have long known that feed atomization is a problem. Although the problem is generic to any process where a liquid must be injected into a vapor phase, several specific problems merit special attention.
As an example, in FCC units the problem is contacting tons per hour of hot regenerated cracking catalyst with large volumes of heavy oil feed. Atomization of the heavy liquid feed into small droplets is needed to ensure complete vaporization of the feed in the base of the riser reactor. Solving this problem has become more difficult with the use of heavier feeds in FCC units. Many FCC's now process liquid feeds with 5-20 wt % resid or non-distillable material. These materials are almost impossible to vaporize in fractionators, so vaporizing them in less than a second or so in an FCC riser reactor is a formidable task.
Liquid feed atomization can also be a problem when adding a heavy liquid to a fractionating column, a hydroprocessing reactor and the like. Because the FCC process is used in most refineries, and the problem of feed atomization is especially acute in such units, the following discussion centers on FCC. It should be understood however, that the problem goes well beyond FCC, and the nozzle which I have developed is useful in many processes besides FCC.
Feed nozzles which were satisfactory for adding a readily vaporizable feed, such as a gas oil, are no longer adequate for heavier feeds. The problems are twofold: the heavier feeds are harder to vaporize because of their high boiling points, and the heavy feeds are harder to atomize because of their high viscosity even at the high temperatures existing in FCC riser reactors.
Efforts of refiners to cope with heavier feeds, or improve the vaporization of lighter ones, will be briefly reviewed.
Some of the efforts at improving regenerated catalyst/feed contacting were on the catalyst side, i.e., the use of lift gas to smoothly lift catalyst up into the riser. Other approaches assumed that catalyst distribution will be poor and forced oil distribution (via multiple nozzles) to be equally poor.
Increased steam addition is common practice for dealing with heavier feeds. Increased atomization steam usually leads to increased pressure drop across the existing feed nozzles, and increased sour water production. Although some improvement in feed dispersion is usually achieved, the problems of increased sour water production, and limits on pressure at which feed can be delivered to the nozzle inlets, limit the improvement from merely increasing steam rate.
Some nozzle designs were developed which required high oil pressure drops across the nozzle for effective operation. Many can only supply the oil feed at relatively low pressure. Major capital expenses would be required for these refiners to use such nozzles. Using a high pressure liquid feed also consumes a considerable amount of energy.
An overview of developments in nozzles is presented in Fluid Catalytic Cracking Report: 50 Years of Catalytic Cracking; A. A. Avidan et al, Oil & Gas Journal, Jan. 8, 1990, at page 50. I am a co-author of this report. As stated there, open pipe or slotted, impact, spiral and critical venturi nozzles have all been used.
The open pipe or slotted nozzle gives coarse irregular droplets. It is not well suited for injecting heavy feeds into an FCC riser reactor, but many refiners still use such nozzles.
Critical venturi nozzles, where an oil and steam mixture pass through a venturi section into a larger, intermediate chamber and discharged through a restricted nozzle, can achieve very small droplet sizes. Such nozzles require high liquid pressure drops and develop a narrow spray pattern with a high velocity which can cause mischief in the process unit into which a liquid is injected. Such nozzles are better suited to making snow than injecting heavy viscous oils into refinery units.
A hybrid approach, use of high velocity steam (1000 to 1800 ft/sec) to atomize a low velocity oil stream (20 to 50 ft/sec) was taught in U.S. Pat. No. 3,654,140, which is incorporated by reference. The high velocity steam imparts energy to the low velocity liquid. FIG. 2 of '140 shows oil discharged as a cone of liquid which is broken into droplets by a high velocity steam stream enveloping the cone. The design was an improvement over the nozzle in U.S. Pat. No. 3,152,065, where liquid passed through an annular region about a central steam pipe to contact an expanding steam stream upstream of a restricted opening. Imparting a centrifugal component to the liquid stream probably threw the liquid to the sides of the nozzle, and may have impaired atomization. The liquid went out as a cone and was not impinged by the high velocity steam stream in the central region of the nozzle.
Although there are myriad nozzle designs, many of which are unique and hard to classify, they can be more or less arbitrarily classified as relying on one or more of the following mechanisms for drop formation.
Restriction/Expansion is the most widely used form of FCC feed nozzle. A mixture of 1-5 wt % atomizing steam and the heavy, preheated feed, pass through a slot or circular orifice to form a spray.
Mixing/Expansion involves use of swirl vanes followed by an orifice.
Shearing atomizes liquid by peeling off a thin sheet of the nozzle feed stream which spontaneously breaks up into small droplets. Spiral FCC feed nozzles are examples.
Gas jet nozzles pass an atomizing gas stream through multiple orifices to strike a liquid stream. The Lechler nozzle is a good example of this type of nozzle.
Impingement nozzles atomize by the high velocity impact of a liquid on a solid surface. The Snowjet nozzle is of this type.
Although it might seem possible to simply stack different types of nozzles in series to improve atomization in practice it does not work. The additional stages may or may not improve atomization, but will always increase pressure drop and this alone will usually prevent simple stacking of these unit operations. Many attempts to improve nozzle performance, as by stacking atomizing devices, degrade performance.
In refinery process units the nozzles must be robust and reliable, as run lengths of one or two years or more are common. FCC units have additional constraints. The hydrocarbon feeds are supplied at a certain pressure, usually around 50-200 psig. Because of the large size of these streams, and the cost of energy needed to pump the feed to higher pressures, and site constraints which frequently prevent easy addition of high pressure pumps, it is important to have a nozzle which will work well with low oil pressures.
Medium or high pressure steam is usually readily available, and is a preferred atomization medium, but refiners usually want to minimize its use. Medium or high pressure steam is a valuable commodity in a refinery, and its use fills much of the FCC riser and downstream processing equipment with an inert material. Refiners are also reluctant to use too much steam, or to have too high an exit velocity from the nozzle, because of catalyst attrition, and riser erosion concerns.
An additional constraint for FCC units is that the material exiting the nozzle should contact as much of the catalyst flowing by the nozzle as possible, without carrying through the catalyst to a side portion of the riser.
It is also beneficial if the nozzles used, whether vertical or side mounted, are relatively small, so that flow of hot catalyst up the riser is not disrupted.
It is also important to have a nozzle which is inexpensive to build so that it can be built from readily available materials such as commercially available pipe and tubing. Such nozzles can also be readily serviced. In contrast, nozzles which require a critical venturi section are more difficult to fabricate, just as venturi meters cost more than orifice flow meters. A concern with precisely machined sections is that after several years of harsh service the precisely machined surfaces may erode.
The nozzle in U.S. Pat. No. 5,289,976 and 5,306,418 represents the "state of the art" in heavy liquid feed nozzles, and represents a significant improvement over the nozzles currently in use. This nozzle had several sections in series:
a radial out-to-in initial atomization section,
an impingement plug,
an annular expansion region, and
a nozzle outlet.
This nozzle was very effective, based on the reported tests on atomizing water. I wanted to develop a nozzle which would be simpler and easier to fabricate and which would provide an acceptable alternative to this design. Based on my experiments with vaporizing water/air mixtures, I was able to develop a nozzle which is somewhat simpler and works better.
In my design I retain an initial atomization section, a feature used in many nozzle designs but eliminate the impingement plug and the annular expansion region. One or more internal plates with a few holes drilled in each plate provided excellent atomization. These could be termed a type of off-center orifice plates with multiple holes, which are utterly unlike orifice plates used to measure flows. These plates are easy to fabricate, readily replaced, and provide excellent atomization. They provided the key to a simpler, shorter, and more effective nozzle.